Electromagnetic spectrum

The electromagnetic spectrum is the range of all wavelengths of light, ranging from extremely short wavelengths such as gamma waves to the very longest wavelengths like radio waves. Only a very small portion of the spectrum is visible to the human eye; this portion of the spectrum is often known as the visible spectrum and is responsible for all the colors that humans see. The other types of electromagnetic waves, although not visible, are still important and useful in daily life.

What is an electromagnetic wave

An electromagnetic wave, also known as electromagnetic radiation, is formed when an atomic particle moves, causing a combined electric field and magnetic field to oscillate, creating a combined wave pattern of the two forces. [1] In the wave pattern, the magnetic and electric waves, although combined into one wave, oscillate at right angles of each other. The electric wave oscillates vertically while the magnetic wave oscillates horizontally, but both waves move along a shared axis in the direction the combined wave is moving. [2] Electromagnetic waves are usually measured in terms of frequency, wavelength, and energy, usually depending upon the size of the waves and the reason for the measurement. The longest waves, like radio and microwaves, are generally measured in frequency; the middle range of waves, like infrared, visible light, and ultraviolet, are measured by wavelength; and the shortest wavelengths, like x-rays and gamma rays, are often measured by the amount of energy they produce. Using these different units to measure electromagnetic waves allow scientists to describe the waves without using numbers that are unwieldy or difficult to use in formulas. The frequency of an electromagnetic wave is measured by computing how many times the crest of a wave passes through a point in 1 second, the wavelength of an electromagnetic wave is the distance between two crests of the wave, and the energy of a wave is described by the amount of energy needed to move an electron. [3] Although these measurement conventions are helpful for scientists in reducing number complexity, it is common to use the term wavelengths to refer to the different electromagnetic waves in general.

Types of electromagnetic waves

The different wavelengths of electromagnetic radiation have different properties and uses depending upon their wavelength. The primary categories of radiation are gamma rays, x-rays, ultraviolet, the visible spectrum, near infrared, far infrared, microwaves, and radio waves. When measuring the size of these electromagnetic waves, the metric system is usually used; wavelengths range in size from being small enough to be measured in nanometers (1 billionth of a meter) to meters. Although the electromagnetic spectrum is divided into different categories, the divisions between the categories are not always clear. Because the waves are part of a spectrum, they transition smoothly from one category to another; meaning that the boundaries between the categories are estimates of where the properties of the waves change.

Gamma rays

Gamma rays are the smallest wavelengths of electromagnetic radiation. Extremely hot objects that release enormous amounts of energy create gamma waves. Supernovas, neutron stars, and pulsars are all natural sources of gamma radiation. Some man-made sources of gamma rays include nuclear explosions, lightning, and some radioactive decay. Gamma waves are so small that they are not even be reflected in a mirror because they can pass through the space between atoms. Special instruments are needed to detect them. Bursts of gamma radiation are so powerful that even a 10 second burst releases more energy than the Sun will in its entire life. [4] Gamma radiation is also dangerous to humans, causing damage to tissue cells. However, gamma rays can also be used to treat some cancers, when used extremely carefully in small, closely monitored doses. [5]

X-rays

X-rays are the next largest wavelength, longer than gamma rays, but smaller than ultraviolet rays; they range between 0.03 and 3 nanometers. Like gamma rays, they are also produced by extremely hot objects. The corona of our sun produces large quantities of x-rays and they can be found in supernova remnants as well. The most common use for x-rays is detecting broken bones and foreign objects within human bodies. The dense material of bones absorbs more x-rays than the surrounding areas, allowing the bones to be viewed by doctors. [6]

Ultraviolet rays

Ultraviolet (UV) rays are the range wavelengths just shorter than the visible spectrum and longer then the x-rays. They have wavelengths of about 10 to 380 nanometers; they can also be notated in micrometers, where they range from 0.01 to 0.38 micrometers. Scientists often classify UV rays into three categories: UV-A, UV-B, and UV-C. The UV-C rays are the most harmful to humans; however, they are absorbed by gases in the atmosphere. The UV-B rays are the primary cause of sunburn, and exposure can cause cellular damage. [7]

Visible spectrum

The visible spectrum is the range of light that the human eye is capable of perceiving. Each color humans see is simply a slightly different wavelength of light reflecting off an object. The visible spectrum ranges from about 400 to 700 nanometers, or 0.4 to 0.7 micrometers. Violet is the shortest wavelength (400 nm) while red is the longest wavelength (700 nm). One use for visible light, aside from simply seeing the world, is that it can be used to measure altitude, or height, from some remote sensing platforms. [8]

Infrared

Infrared waves are larger than the visible spectrum and smaller than microwaves. Their part of the spectrum is relatively large compared to the previous categories, ranging from about 0.7 micrometers (700 nanometers) to 1000 micrometers. [9] The infrared spectrum is usually divided into near, mid, and far infrared. Near-Infrared (NIR) begins just beyond the red portion of the spectrum at 0.7 micrometers to about 3 micrometers. This region is often used in remote sensing to identify vegetation, especially healthy vegetation, because plants strongly reflect this band of light (see Remote Sensing). The mid-infrared, consisting of wavelengths from around 3-15 micrometers, contains the region of the spectrum also known as thermal infrared. Thermal infrared is used to detect heat, especially objects that are not hot enough to give off visible light. Infrared can also be used to see through dense clouds, both on earth and in space. This helps scientists studying the earth because clouds are so prevalent and it can be difficult to study the earth below the clouds when it is obscured by cloud cover. Astronomers use infrared to see through nebulas and study star formation. Stars are often formed within the dense clouds of gas and dust that form nebulas, and the longer wavelengths that comprise infrared waves pass through the obstructions and allow scientists to view the newly forming stars. [10]

Microwaves

Microwaves are the range of wavelengths longer than infrared and shorter than radio waves; the wavelengths range from 1000 micrometers to 1 meter. Like infrared, microwaves are used in remote sensing to see through cloud cover. They are also used in Global Positioning Systems (GPS), meteorology, and cooking food. The long waves of the microwave spectrum can study the earth even through dense clouds while the shorter end of the spectrum is used in microwave ovens to heat food.

Radio waves

Radio waves usually consist of electromagnetic waves longer than 1 meter. Some of the waves are even the size of the planet. Some uses for Radio waves include communication, remote sensing, and astronomy. [11] The AM and FM bands used in the average radio are different wavelengths in the radio wave spectrum, which can transport information over long distances.

Atmospheric absorption

Not all bands of electromagnetic radiation pass through the earth’s atmosphere to reach the surface. The gases in the atmosphere absorb certain wavelengths of light and restrict their passage through the atmosphere while allowing other wavelengths to pass through easily. The wavelength bands that are restricted by the atmosphere are known as absorption bands while the regions of the spectrum that are allowed through are known as atmospheric windows. Gamma rays, x-rays, parts of the ultraviolet spectrum, and much of the infrared spectrum are all in absorption bands and do not reach the earth’s surface, while some ultraviolet rays, the visible spectrum, parts of the infrared spectrum, microwaves, and some radio waves reach the earth's surface with relative ease.[12] Atmospheric absorption can make studying parts of the electromagnetic spectrum difficult, especially when studying objects in space, because the waves cannot be studied from the surface of the earth. In these cases, satellites in earth's orbit are calibrated to detect specific parts of the spectrum and then send the information to scientists on earth.

Blackbody curves

A blackbody curve is a theoretical construct of the total amount of radiation an object emits. It is usually depicted as a graph with all the wavelengths the object emits along the x-axis and the amount of energy emitted by each wavelength along the y-axis. The curve created by the graph depicts the total amount of energy emitted by the object across all wavelengths of light. Any object with a temperature above absolute zero (0 degrees Kelvin) emits some amount of energy and light. The temperature of the object is directly related to the amount of energy it produces and its peak wavelength. The peak wavelength of an object is the wavelength at which the most energy is emitted. The sun’s peak wavelength is in the visible spectrum, while the earth’s peak wavelength is in the infrared spectrum. The more energy an object produces, the shorter it’s peak wavelength. That is why high energy, very hot objects like supernovas and neutron stars produce a lot of electromagnetic waves from the shortest end of the spectrum, such as gamma rays and x-rays. Cooler objects that emit lower amounts of energy have wavelengths that peak in the longer part of the spectrum, such as infrared or microwaves. Thus, the energy an object emits is related to its peak wavelength and its temperature to such a point that scientists can determine the temperature of an object from its peak wavelength. However, blackbody curves are considered theoretical because a true blackbody, i.e. a perfect emitter or absorber of energy, does not exist. Fortunately, many objects, such as stars, are very close to being blackbodies and behave approximately as a blackbody would, which allows scientists to use the concept of blackbodies in their studies. [13][14]

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